a. Field of the Invention
The instant invention relates to the treatment of atrial fibrillation. In particular, the instant invention relates to devices and methods for visualizing target tissues during epicardial mapping and ablation procedures.
b. Background Art
It is well known that atrial fibrillation results from disorganized electrical activity in the heart muscle (the myocardium). The surgical maze procedure has been developed for treating atrial fibrillation, and involves the creation of a series of surgical incisions through the atrial myocardium in a preselected pattern so as to create conductive corridors of viable tissue bounded by scar tissue.
As an alternative to the surgical incisions of the maze procedure, transmural ablations of the heart may be used. Such ablations may be performed either from within the chambers of the heart (endocardial ablation), using endovascular devices (e.g., catheters) introduced through arteries or veins, or from outside the heart (epicardial ablation) using devices introduced into the patient's chest. Various ablation techniques may be used, including, but not limited to, cryogenic ablation, radio frequency (RF) ablation, laser ablation, ultrasonic ablation, and microwave ablation. The ablation devices are used to create elongated transmural lesions—that is, lesions extending through a sufficient thickness of the myocardium to block electrical conduction—forming the boundaries of the conductive corridors in the atrial myocardium. Perhaps most advantageous about the use of transmural ablation rather than surgical incision is the ability to perform ablation procedures without first establishing cardiopulmonary bypass (CPB).
In performing the maze procedure and its variants, whether using ablation or surgical incisions, it is generally considered most efficacious to include a transmural incision or lesion isolating the pulmonary veins from the surrounding myocardium. The pulmonary veins connect the lungs to the left atrium of the heart, joining the left atrial wall on the posterior side of the heart. Such procedures have been found to offer 57% to 70% success without antiarrhythmic drugs. However, they are also associated with a 20% to 60% recurrence rate as the result of lesion recovery, non-pulmonary vein foci of the arrhythmia, or the need for further tissue modifications.
Previous surgical and catheter-based approaches have demonstrated that linear left atrial (LA) lesions were successful in treating atrial fibrillation when complete block was achieved. One such technique involves linear ablation at the mitral isthmus, which is defined as extending from the lateral mitral annulus to the ostium of the left inferior pulmonary vein (LIPV). Studies have shown that catheter ablation of the mitral isthmus, in combination with pulmonary vein (PV) isolation, consistently results in demonstrable conduction block and is associated with a high cure rate for paroxysmal atrial fibrillation.
Producing precise lesions at these locations presents significant obstacles for the physician performing endocardial ablations for several reasons. First, while many of the lesions created in the maze procedure can be created from within the right atrium, the pulmonary venous lesions must be created in the left atrium, requiring either a separate atrial access point or a transseptal puncture from the right atrium. Second, the elongated and flexible endovascular ablation devices are difficult to manipulate into the complicated geometries required for forming the pulmonary venous lesions. It is also difficult to maintain proper positioning of the ablation device against the wall of a beating heart. Furthermore, visualization of endocardial anatomy and endovascular devices is often inadequate, such that knowing the precise position of an endovascular device can be difficult, potentially resulting in misplaced lesions.
Epicardial ablation devices and methods useful for creating transmural lesions for the treatment of atrial fibrillation have been described in U.S. Pat. No. 7,052,493 to Vaska et al. (“Vaska”) and U.S. Pat. No. 6,971,394 to Sliwa et al. (“Sliwa”), both of which are hereby expressly incorporated by reference as though fully set forth herein. Sliwa describes a method of forming a transmural lesion in a wall of the heart adjacent to the pulmonary veins by placing an ablation device through a thoracic incision, and then through a pericardial penetration, so that the ablation device is disposed in contact with an epicardial surface of the heart. The ablation device includes a locating device, such as a catch, a branch, or a notch, near the working end of the catheter that is configured to engage one or more of the pulmonary veins or another nearby anatomical structure (e.g., a pericardial reflection, the inferior vena cava, the superior vena cava, the aorta, the left or right atrial appendage) in order to position the working end of the catheter adjacent to the pulmonary veins.
Precise placement of the ablating elements on the epicardial surface is critical to the efficacy of PV isolation and mitral isthmus ablations. The anatomic relationship of tissues other than the heart is also a very important consideration. For example, due to the proximity of the esophagus to the posterior left atrium of the heart, great care must be taken to prevent damage to the esophagus during an ablation procedure, as atrioesophageal fistulas have been reported as a complication following catheter ablation for atrial fibrillation. Reliance on non-visual locating devices, however, requires a very high degree of physician skill and experience, and may hinder such precise placement.